1. Field of the Invention
The present invention relates to the field of cleaning a workpiece surface. More particularly this invention relates to the area of megasonic cleaning of a semiconductor wafer.
2. Discussion of Related Art
The increasing process complexity and introduction of new materials at the 45 nm and 32 nm technology nodes has given rise to a greater number of processing steps and correspondingly to the number of cleaning steps required in order to maintain yields. Accordingly, wafer cleaning has become the most frequently repeated operation in IC manufacturing. One of the main goals is to remove contaminants such as particles from the wafer surface and additionally from other semiconductor workpiece surfaces such as lithography tools, including patterned lithography masks.
Cleaning operations have traditionally concentrated on the front end of the line (FEOL) processing where active devices are exposed and detailed cleans are required. On the device side of the wafer, it is necessary to remove contaminants which could cause a malfunctioning device. Contamination on the non-device side (backside) of the wafer can also cause a number of problems. For example, backside particle contamination can cause a photolithography step on the front side to be out of focus, which can cause significant plant downtimes. In other occasions, a deposition tool deposits a material on the front side on the wafer but inadvertently also deposits some film on the backside of the wafer. In copper electroplating tools, for example, copper contamination can end up on the backside of the wafer. In all these cases, the backside has to be cleaned of particles and/or dissolved metals or certain layers have to be stripped.
The RCA clean, a wet-chemical cleaning method developed in 1965, still forms the basis for most FEOL wet cleans. A typical RCA-type cleaning sequence comprises a first alkaline standard clean (SC1) solution, commonly containing hydrogen peroxide (H2O2) and ammonium hydroxide (NH4OH) in deionized water for removal of organic and particulate contaminants, followed by a second standard clean (SC2) solution, commonly containing hydrogen peroxide and hydrogen chloride (HCl) in deionized water for removal of metallic impurities. Often a high pH and temperature of the SC1 solution is required to remove particles. This has a detrimental effect of etching the wafer surface and can be too aggressive for many critical cleaning steps.
The recent introduction of megasonics assisted cleaning techniques has allowed for better particle removal efficiency with cleaning solutions that are substantially less aggressive and thus less harmful to the surface of the wafer. Megasonic energy is typically applied to a cleaning solution by sonicating the cleaning solution with high energy acoustic waves. The cleaning ability of megasonic energy is largely attributed to various combinations of cavitation and acoustic streaming. Acoustic streaming is the result of the high energy acoustic waves propagating through the cleaning solution. Acoustic streaming generally lifts off particles off surfaces with shear stresses. Cavitation includes gas bubble oscillation and implosion in the cleaning solution resulting from the pressure changes of acoustic waves moving through the cleaning solution. Cavities may form when the liquid pressure momentarily drops below the vapor pressure during the low pressure portion of the acoustic wave. Typically the cavities preferentially form at the site of imperfections (for example, dissolved gas bubbles) in the liquid which serve as nuclei for cavitation. When a high enough pressure amplitude, called cavitation threshold, is reached the nucleus becomes unstable and rapidly grows into a transient cavity which may then violently implode. Bubble implosion near the wafer surface can be a significant contributor to particle removal. However, the violent implosions associated with cavitation can also destroy fragile patterns on the wafer device side.
Various methods have been developed in an attempt to remove particles from a wafer backside without damaging the fragile patterns on the wafer front device side. One such proposal is to clean the backside of a wafer by exposing only the backside of the wafer to megasonic energy. However, at high powers associated with megasonic cleaning, energy propagates from the wafer backside, through the wafer, and to the wafer device side and is capable of damaging fragile structures on the device side. Additionally, the energy that propagates to the wafer device side is capable producing cavitation in the cleaning solutions on the wafer device side.
Another proposal has been to reduce the size of cavities that form and implode on the device side by increasing the megasonic frequency. As megasonic frequency is increased, the amount of time between pulses is shortened resulting in smaller cavitation bubbles which in turn produce less violent collapse. However, raising the megasonic frequency, while resulting in “controlled” cavitation on the device side, also has the detrimental effect of reduced cleaning efficiency on the wafer backside, particularly for larger particles. Even more, while previous technology nodes utilized patterns on the device side, such as poly-lines, that could withstand the less violent implosions associated with “controlled” cavitation, patterns in the 45 nm and 32 nm nodes are becoming so fragile that megasonic agitation may break them and cause a defective device.
Therefore, what is needed is a cleaning method that is capable of removing particles from the backside of a semiconductor workpiece (wafer, mask, etc.) while at the same time also eliminating unwanted etching and damage on the fragile device side of the semiconductor workpiece.